In semiconductor processing, multiple lithography steps are generally used to form a semiconductor chip. These steps typically include forming a photoresist over a substrate that is to be formed into the semiconductor chip, exposing the photoresist to a pattern of light generally controlled by a mask, developing the pattern in the photoresist to expose the underlying substrate, and etching the pattern in the underlying substrate. The pattern etched in the underlying substrate may be a basis for some feature formation, such as an ion impurity implantation such as for doping source and drain regions, a formation of a structure like a gate patterning, or a pattern for a conductive material such as in a metallization layer.
Advances in semiconductor processing have generally allowed for continued reduction of minimum feature sizes for semiconductor chips; however, each reduction in size typically is accompanied with its own challenges. As minimum feature size decreases, overlay concerns are typically intensified. Minimum feature size reduction allows for a greater density of features in a given area, which in turn increases the likelihood that an improperly overlaid feature can render the chip useless.
To help avoid improper overlaying of features, critical dimensions of features typically must be precise. If dimensions of features are just a small amount more than corresponding critical dimensions, the features can overlay other features. Accordingly, it is advantageous to form features with precise dimensions that are true to a design.
As previously noted, lithography steps are typically used to form these features, and at the source of the lithography steps is the mask. If the mask is not able to precisely form a pattern in a photoresist, the subsequently formed feature may not meet its critical dimension requirement. Some causes for a mask not being able to precisely form a pattern may be that the pattern formed in the mask itself is not precise, such as caused by a slight, unintended etching of material holding the pattern in the mask during the cleaning of the mask while being formed. This slight etch may widen the pattern in the mask beyond a critical dimension. Further, various physical characteristics of the mask, such a reflectivity percentage, a refractive index, and a coefficient of extinction, may cause an exposure through the mask to be improperly focused or allow spurious light to be incident on the photoresist.